Is there an optimal repair point?

Yes, it's condition 70%, according to this: http://web.archive.org/web/20120603154301/http://www.dead-island.com/forums/dead-island-tips-tricks-guides/4266-optimal-repair-cost.html.

• The repair cost does not increase by a static amount for every hit.
• The repair cost seems to be about 3.6644 times the default value of the weapon when durability is depleted.
• The increase in repair cost per hit follows an inverted parabolic curve.
• The durability is, in most cases, one less than the number of hits a weapon can inflict before breaking.
• There seems to be a base repair cost which is about 33% of the value, thereby making the first hit always the most expensive.
• Summary: The optimal repair point is 70% durability. The cost per hit is most expensive at 40% durability.